The “thermally foamable microsphere” also referred to as a thermally expandable microcapsule is obtained by microencapsulating a volatile foaming agent with an outer shell formed of a polymer. In general, when suspension polymerization with a polymerizable mixture containing a polymerizable monomer and a foaming agent is allowed to proceed in an aqueous dispersion medium, an outer shell is formed in such a manner as to encapsulate the foaming agent.
As the polymer forming the outer shell, a thermoplastic resin with good gas barrier properties is generally used. The polymer forming the outer shell is softened with heat. As the foaming agent, a compound having a low boiling point, such as hydrocarbon, which becomes gas at a temperature equal to or lower than the softening point of the polymer forming the outer shell is generally used.
When the thermally foamable microsphere is heated, the foaming agent is vaporized to generate expanding force acting on the outer shell. Simultaneously, the elastic modulus of the polymer forming the outer shell drastically decreases, which causes rapid expansion at a certain critical temperature. This temperature is referred to as a “foaming starting temperature.” When the thermally foamable microsphere is heated to a temperature equal to or higher than the foaming starting temperature, foamed particles (expanded and closed cells) are formed due to the expansion phenomenon. When further heated, the foaming agent transmits the outer shell, which has become thin, resulting in reduced internal pressure, which causes contraction of the foamed particles (shrinking phenomenon).
Utilizing the above-described properties which allows the formation of the foamed particles, the thermally foamable microsphere is applied to a wide variety of fields, such as a designability providing agent, a functionality providing agent, and a weight reducing agent. For example, the thermally foamable microsphere is added to a polymeric material, such as a synthetic resin (a thermoplastic resin and a thermosetting resin) and rubber, a coating composition, ink, etc., for use. When higher performance is required in each field to which the microsphere is applied, the demand level to the thermally foamable microsphere increases. Thus, for example, the improvement in the processing properties, such as heat resistance, is required.
However, in a conventional thermally foamable microsphere, the foaming starting temperature is generally within a narrow range and foaming starts at relatively low temperatures. Therefore, the thermally foamable microsphere is likely to develop premature foam at the time of processing, such as kneading or pelletization before foamed molding. Therefore, the processing temperatures need to be low, which limits the type of an applicable synthetic resin or rubber.
Conventionally, in order to obtain a thermally foamable microsphere usable at high temperatures, a thermally foamable microsphere has been proposed: which each comprises: a shell of a polymer obtained by the polymerization of acrylonitrile (I) as the main monomer, a monomer (II) having a carboxyl group, and a monomer (III) having a group reactive with the carboxyl group of the monomer (II); and encapsulated therein a liquid having a boiling point not higher than the softening point of the polymer (Patent Document 1). The foamed article obtained by the method has a feature of having a glass-like fragile outer shell. For this reason, the foamed article is completely different from one having elasticity. Thus, the properties of resin may be lost when producing a porous body whose shape varies.
Moreover, Patent Document 2 proposes a method of forming an outer shell resin of a thermally foamable microsphere from a polymer of a monomer mixture containing a nitril monomer (I), a monomer (II) having one unsaturated double bond and a carboxyl group in the molecule, a monomer (III) having two or more polymerizable double bonds in the molecule, and, as required, a monomer (IV) which can be copolymerized with the monomers. According to this method, the heat resistance can be increased. However, by the use of a monomer having two or more polymerizable double bonds in the molecule, a polymer takes a cross linkage structure, whereby the expansion ratio is suppressed. When acrylonitrile is used in a high proportion, aggregation occurs to form a lump in the middle of polymerization, which makes it difficult to secure manufacturability. Moreover, when acrylonitrile is used in a high proportion, yellowing is remarkable on heating.
Conventionally, polymethacrylimide is known as a polymeric material having high heat resistance, and a polyimide foam substance using such material is disclosed in Patent Document 3. The production method disclosed in Patent Document 3 refers to a method of heating and foaming a polymer plate after producing a foam substance, and does not refer to a method of producing a thermally foamable microsphere.    Patent Document 1: International Publication WO 99/43758    Patent Document 2: International Publication WO 03/099955    Patent Document 3: Japanese Laid-open Publication No. 10-306169